Image 1
If you look at image 1, you can see a row of f/numbers on the top of the cart. They are f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22. These f/numbers represent the aperture size or aperture opening. The bigger f/number gives the smaller aperture opening. Thus from this sequence of numbers f/1.0 gives biggest aperture size and f/22 gives the smallest aperture.
On the left side of the chart there is a column of numbers that represent shutter speeds. It tells how long the light will go through the aperture. They are 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, 1/500 sec. Although the numbers are getting bigger from top to bottom, in fact the time is getting shorter. Thus the 1/8 sec is longer than 1/15 sec, but 1/250 is shorter than 1/125 sec.
Every step in the chart from left to right and from top to bottom is called one stop. It is also applicable for every step from right to left and from bottom to top. For example, if you want to set up f/numbers from f/2.0 to f/2.8, you have to move one stop, but if you want to set up f/number from f/4.0 to f/8.0 you have to do two stops. You have to keep the same approach to the shutter speed column. Increasing time in one stop you have to change your shutter speed from 1/8 sec to 1/4 sec, but decreasing the shutter speed in one stop you have to move from 1/125 sec to 1/250 sec and so on. The movement from 1/8 sec shutter speed to 1/125 sec give you 4 stops.
You can ask, why do I need this? OK. Have a look at an example from real life situation. I was walking with my friend on countryside and doing her portraits. For portraiture is better to use a wide opening aperture. It gives a focus on the subject, but not on the background. For this reason I used the f/2.8 number and the shutter speed 1/125 sec. Then I decided to take photos of landscapes that surrounded us. For landscapes is better to use small aperture size such as f/5.6, f/8.0, f/11, f/16 and f/22. I chose the f/5.6. It meant I made 2 stops from f/2.8 to f/5.6 towards right side of the chart. To keep the same amount of exposure I have to move from 1/125 sec to 1/30 sec shutter speed. By another word I have to move also 2 stops towards the top of the shutter speed column.
If you look at the chart you can see that a combination f/2.8 and shutter speed 1/125 sec gives a cell with number 10. If you do two stops on the right and towards the top of the chart, you will get the number 10 again.
Thus the chart can be very useful, if you do not have time to figure out how to find the same exposure. Only you have to do is look at the chart and find the same number that a previous setting gave you. In my case it was the number 10.
Useful Links:
Exposure